It has long been common practice to use hydantoin and substituted hydantoins as precursors and intermediates in the synthesis of amino acids. The use of substituted hydanotins in the synthesis of amino acids such as alanine, methionine, tryptophan and lysine is well documented in the prior art. (Kirk Othmer, Encyclopedia of Chemical Technology, Volume 12, pages 694-695). The recent development of the artificial sweetner aspartame has also focused attention on the use of 5-arylidene substituted hydantoin as an intermediate in the synthesis of phenylalanine, a necessary ingredient in the synthesis of aspartame. The process for carrying out the condensation reaction of an aromatic aldehyde with hydantoin to form these 5-arylidene substituted hydantoins is also well known. (Wheeler and Hoffman, Amer. Chem. J., Volume 45, pages 368-83 (1911). A number of patents report improvements on these methods.
In U.S. Pat. No. 2,861,079, unsaturated hydantoins are produced by reacting aldehydes with hydantoin in an aqueous solution or a solution of a lower aliphatic alcohol containing an equimolar amount of a monoalkanolamine. This method has the disadvantage that, even when water is used as a solvent, large quantities of water soluble expensive amines, such as diethanolamine, are required.
In U.S. Pat. No. 4,345,072, an aromatic aldehyde substituted or unsubstituted in the aromatic nucleus is reacted with hydantoin in the presence of an equimolar amount of at least one ammonium salt of an aliphatic or aromatic carboxylic acid. A disadvantage of this method is that it requires the use of expensive carboxylic acids as solvents. Even if water is used as the solvent, this patent still requires the use of expensive ammonium salts of carboxylic acids. In addition, the process of the U.S. Pat. No. '072 requires the use of a high molar ratio of catalyst to hydantoin to achieve the desired results.